1. Field
Embodiments herein generally to the operation of dual-riser fluidized catalytic cracking (FCC) units to produce olefins and/or aromatics from light hydrocarbon feedstocks, and in particular from feedstocks rich in C3 and/or C4 hydrocarbons.
2. Description of the Related Art
Fluid catalytic cracking (FCC) technology has been employed for more than 50 years in refineries to maximize yields for transportation fuels such as gasoline and distillates. The FCC process uses a reactor called a riser which is essentially a pipe in which a hydrocarbon feed gas is intimately contacted with small catalyst particles to effect the conversion of the feed to more valuable products. The FCC unit typically converts gas oil feeds by “cracking” the hydrocarbons into smaller molecules. The resulting hydrocarbon gas and catalyst mixture both flow in the riser, hence the term fluid catalytic cracking.
As employed in today's refineries, the FCC unit has found use in converting primarily heavy feeds, such as vacuum gas oils, reduced crudes, atmospheric tower bottoms, vacuum tower bottoms and the like, into more transportation fuel products such as gasoline, diesel, heating oils, and liquefied petroleum gases. To increase yields from the FCC unit of more valuable petrochemicals, such as ethylene and propylene, refineries are operating at high severity and/or using light feedstocks such as light cracked naphtha in the riser to co-crack with heavy feeds.
The cracking reaction is endothermic, meaning that heat must be supplied to the reactor process to heat the feedstock and maintain reaction temperature. During the conversion process with heavy feeds, coke is formed. The coke is deposited on the catalyst and ultimately burned with an oxygen source such as air in a regenerator. Burning of the coke is an exothermic process that can supply the heat needed for the cracking reaction. The resulting heat of combustion from regeneration increases the temperature of the catalyst, and the hot catalyst is recirculated for contact with the feed in the riser, thereby maintaining the overall heat balance in the system. In balanced operation, no external heat source or fuel is needed to supplement the heat from coke combustion. Should a heat imbalance exist, such as making too much coke and generating excessive heat for the reactions, it is possible to use a catalyst cooler or other process modifications in mitigation, especially with heavy feeds or high severity operation. As practiced today, the FCC unit primarily cracks gas oil and heavier feeds.
The prior art teaches the conversion of light feeds such as C4+ olefinic and paraffinic streams to more valuable products such as propylene. This process technology is commercially available under the trade designation SUPERFLEX. The processing of light feeds, generally with carbon numbers less than 12, poses its own unique issues with regards to two critical areas, namely maximizing the propylene and ethylene yields, and maintaining the heat balance with insufficient coke make. These issues become even more important as lighter feeds are contacted with catalysts formulated specifically for light feeds and higher ethylene and propylene production.
Unlike heavy feeds, light feeds do not make enough coke to maintain heat balance in the FCC unit. Thus, an external source of heat input is required to keep the FCC unit in heat balance when using predominantly light feeds. U.S. Pat. No. 7,011,740 teaches the use of an import fuel oil to remove catalyst fines from the riser reactor effluent, and combusting the imported fuel oil to heat balance the FCC unit. Special regenerators, for the continuous firing of fuel in the regenerator for use in an FCC unit with a light feed, are known (as an example, see commonly assigned U.S. patent application Ser. No. 10/065,376, filed Oct. 10, 2002 (Publication US 20040069681)).
To maximize the utilization of low value feeds within a refinery or petrochemicals complex, producers have introduced much lighter feeds into the FCC unit. Lighter feeds require a hotter riser temperature to crack efficiently, but when introduced in a small proportion into a heavy feed stream, will lead to even more coke production. This occurs because although the coke make from lighter feeds is significantly lower than for heavy feeds at the same temperature, the coke make from the heavy feed is increased at the higher operating temperatures. Conditions that maximize the production of propylene generally require relatively high temperatures that increase coke production, particularly from the heavy feed. Light feeds rarely make 1 percent coke, while the coke yield from heavy feeds could be as high as 10-15 percent. The excess coke from heavy feed under propylene-maximizing conditions would generally lead to a system heat imbalance, unless a catalyst cooler were used.
In the prior art, use of the excess heat from the coke formed in the heavy feed riser to supply the heat of reaction required by the lighter feed supplied to a second riser is known to be generally more efficient (See Eng et al., “Economic Routes to Propylene,” Hydrocarbon Asia, p. 36 (July/August 2004), which discloses the production of transportation fuels from a heavy feed such as vacuum gas oil in a conventional FCC unit as a baseline). However, if the goal is to maximize petrochemicals, the FCC unit can use both heavy and light feeds. A variation on the SUPERFLEX process is the use of a dual riser reactor known under the trade designation SUPERFLEX PLUS. In the dual riser process, a light feedstock is supplied to one riser to produce the olefins that are desired, while a conventional resid or heavy feedstock is supplied to another riser to make gasoline and/or distillates. The catalyst from the dual risers is regenerated in a common regenerator. The heat from regenerating the coke deposits, primarily on the catalyst from the heavy feed riser, is balanced for operation of both risers. Because optimum cracking conditions for the heavy feed and light feed are usually much different, this paper teaches that the complete segregation of a heavy feed from a light feed cracked in dual risers will lead to benefits in yields and operation.
Integration of gas oil and light olefin catalytic cracking zones with a pyrolytic cracking zone to maximize efficient production of petrochemicals is known (See commonly assigned U.S. Pat. No. 7,128,827). Integration of the units in parallel allows production of an overall product stream with maximum ethylene and/or propylene by routing various feedstreams and recycle streams to the appropriate cracking zone(s), e.g. ethane/propane to the steam pyrolysis zone, waxy gas oil to a high severity cracking zone and C4-C6 olefins to the light olefin cracking zone, enhancing the value of the material balances produced by the integrated units.
Processes for catalytically and non-catalytically cracking hydrocarbon feedstocks are well known. Steam cracking in a furnace and contact with hot non-catalytic particulate solids are two well-known non-catalytic cracking processes. Exemplary processes are described in U.S. Pat. Nos. 3,407,789; 3,820,955; 4,499,055; and 4,814,067. Fluid catalytic cracking and deep catalytic cracking are two well-known catalytic cracking processes. U.S. Pat. Nos. 4,828,679; 3,647,682; 3,758,403; 4,814,067; 4,980,053; and 5,326,465 disclose exemplary processes.
Zeolite-based heterogeneous catalysts are used by industrial chemical companies in the interconversion of hydrocarbons in an FCC unit for example, and in the alkylation of aromatic compounds. A very good example is the zeolite ZSM-5. This zeolite, developed by Mobil Oil, is an aluminosilicate zeolite with a high silica and low alumininum content. Its structure is based on channels with intersecting tunnels. The aluminium sites are very acidic. The substitution of Al3+ in place of the tetrahedral Si4+ silica requires the presence of an added positive charge. When this is H+, the acidity of the zeolite is very high. The reaction and catalysis chemistry of the ZSM-5 is due to this acidity.
While much of the prior art directed to light hydrocarbon processing in FCC reactors focuses on improved olefin yields, there remains a need for improving the yield of aromatics, especially from low value light hydrocarbon feedstocks such as liquefied petroleum gas (LPG).